There are numerous applications where chemicals, dissolved or suspended in a fluid, are required to be applied over large areas or volumes.
One such application is in fire fighting, where fire retardant foam may sometimes be employed to smother a fire, particularly in the case of electrical fires or flammable liquid fires (such as oil fires) in which water is not a suitable fire extinguishing material.
The chemical components of the fire retardant foam are typically dissolved or suspended in water, the foam readily forming as the water and chemicals are sprayed out of a nozzle. The primary component of the fire retardant foam is a surfactant, which readily foams when sprayed out of the nozzle. The concentration of the surfactant in the water is typically less than 1%. Some types of surfactants employed include synthetic surfactants such as alpha-olefin sulfonates, perfluorooctane sulfonate, perfluorooactanoic acid and protein based surfactants. Other components of fire retardant foams may include organic solvents such as trimethyltrimethylene glycol and hexylene glycol, foam stabilisers such as lauryl alcohol and corrosion inhibitors.
For hand held fire extinguisher cylinders, the foam components and water are held in a compressed volume, similar to the arrangement in an aerosol can. When operated, the compressed gas which compresses the water and foam components escapes through a valve, causing the application of a pressure on the water, driving it out of the cylinder through a nozzle. As this occurs, the foaming components form the foam.
For fire trucks, having a foam system, a foam tank containing water and the foam components is provided on board the fire truck. In this instance, however, the pressure required to drive the water carrying the foam components through a nozzle to form the foam is provided by the fire truck's onboard pump.
Although both these arrangements enable an operator to apply a controlled concentration of chemicals to produce a fire retardant foam, a significant disadvantage is that large volumes of water and these chemicals are required to be stored.
In more recent developments, solid blocks containing surfactants supported in a polyethylene glycol matrix have been employed to create fire retardant foams by placing the solid blocks in the path of a stream of water. Typically, the block is housed in a chamber at some point between a water source and a nozzle. As the stream of water passes around the block in the chamber, the block including the surfactant dissolves and/or erodes into the fluid. The rate of loss of material from the block is directly proportional to the exposed surface area of the chemical block. The fluid, now containing surfactant, may flow through a nozzle to create the fire retardant foam. A significant advantage of this arrangement is that it is not necessary to provide a large storage tank of water containing the foaming compounds. However, a noticeable problem in relation to such solid blocks is that the loss of material from the blocks into the fluid does not occur evenly over the surface of the block over time. It has been found that this problem causes a rapid reduction in the concentration of foaming agents in the fluid. It is consequently necessary to regularly replace the solid block in the chamber, without a substantial portion of the solid block having been used, in order to maintain the required concentration.
Another application where chemicals dissolved or suspended in a fluid need to be applied over a large area or volume is in the application of agricultural products such as fertilisers, pesticides, insecticides and herbicides. Such chemicals may be made up into a solution and sprayed over the necessary area using an irrigation system or a hose for example. Again, one problem with this arrangement is that it requires storage of large volumes of chemical containing solution. Furthermore, such chemicals can be difficult to handle when in liquid form.
In a further application, cleaning apparatus for cleaning cars and other vehicles may have accessories which enable a liquid detergent concentrate to be added to a stream of water to assist in the cleaning process. The detergent is added by flowing the water past an opening to a volume of the detergent through the opening and, which draws some of the detergent out into the water under a “venturi” effect.